Etch stop layer for silicon (Si) via etch in three-dimensional (3-D) wafer-to-wafer vertical stack

ABSTRACT

A method of forming a silicon (Si) via in vertically stacked wafers is provided with a contact plug extending from selected metallic lines of a top wafer and an etch stop layer formed prior to the contact plug. Such a method comprises selectively etching through the silicon (Si) of the top wafer until stopped by the etch stop layer to form the Si via; depositing an oxide layer to insulate a sidewall of the Si via; forming a barrier layer on the oxide layer and on the bottom of the Si via; and depositing a conduction metal into the Si via to provide electrical connection between active IC devices located on vertically stacked wafers and an external interconnect.

TECHNICAL FIELD

The present invention relates to a semiconductor process and, morespecifically, relates to a method of using Nickel Silicide (NiSi) as anetch stop layer for the silicon (Si) via etch in vertically stackedwafer processing in a three-dimensional (3-D) wafer-to-wafer verticalstack.

BACKGROUND

Integrated circuits (ICs) form the basis for many electronic systems.Essentially, an integrated circuit (IC) includes a vast number oftransistors and other circuit elements that are formed on a singlesemiconductor wafer or chip and are interconnected to implement adesired function. The complexity of these integrated circuits (ICs)requires the use of an ever increasing number of linked transistors andother circuit elements.

Many modern electronic systems are created through the use of a varietyof different integrated circuits; each integrated circuit (IC)performing one or more specific functions. For example, computer systemsinclude at least one microprocessor and a number of memory chips.Conventionally, each of these integrated circuits (ICs) is formed on aseparate chip, packaged independently and interconnected on, forexample, a printed circuit board (PCB).

As integrated circuit (IC) technology progresses, there is a growingdesire for a “system on a chip” in which the functionality of all of theIC devices of the system are packaged together without a conventionalPCB. Ideally, a computing system should be fabricated with all thenecessary IC devices on a single chip. In practice, however, it is verydifficult to implement a truly high-performance “system on a chip”because of vastly different fabrication processes and differentmanufacturing yields for the logic and memory circuits.

As a compromise, various “system modules” have been introduced thatelectrically connect and package integrated circuit (IC) devices whichare fabricated on the same or on different semiconductor wafers.Initially, system modules have been created by simply stacking twochips, e.g., a logic and memory chip, one on top of the other in anarrangement commonly referred to as chip-on-chip structure.Subsequently, multi-chip module (MCM) technology has been utilized tostack a number of chips on a common substrate to reduce the overall sizeand weight of the package, which directly translates into reduced systemsize.

Existing multi-chip module (MCM) technology is known to provideperformance enhancements over single chip or chip-on-chip (COC)packaging approaches. For example, when several semiconductor chips aremounted and interconnected on a common substrate through very highdensity interconnects, higher silicon packaging density and shorterchip-to-chip interconnections can be achieved. In addition, lowdielectric constant materials and higher wiring density can also beobtained which lead to the increased system speed and reliability, andthe reduced weight, volume, power consumption and heat to be dissipatedfor the same level of performance. However, MCM approaches still sufferfrom additional problems, such as bulky package, wire length and wirebonding that gives rise to stray inductances that interfere with theoperation of the system module.

An advanced three-dimensional (3D) wafer-to-wafer vertical stacktechnology has been recently proposed by researchers to realize theideal high-performance “system on a chip”. In contrast to the existingmulti-chip module (MCM) technology which seeks to stack multiple chipson a common substrate, 3-D wafer-to-wafer vertical stack technologyseeks to achieve the long-awaited goal of vertically stacking manylayers of active IC devices such as processors, programmable devices andmemory devices inside a single chip to shorten average wire lengths,thereby reducing interconnect RC delay and increasing systemperformance.

One major challenge of 3-D wafer-to-wafer vertical stack integrationtechnology is the bonding between wafers and between die in a singlechip. In general, a dielectric layer is used to bond respective wafers.In the dielectric bonding method, high aspect ratio silicon (Si) viaetch is required to provide electrical conductivity between verticallystacked wafers. Therefore, metal bonding method is preferred to stackwafers. However, existing etch stop layers for silicon (Si) via etch inwafer stacking with metal bonding method can cause oxidation or metalrecess which may negatively impact the power delivery system. Therefore,a need exists for an improved etch stop layer for the Si via etch invertically stacked wafer processing (i.e., 3-D interconnect processing)which provides electrical conductivity between vertically stackedwafers.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of exemplary embodiments of the presentinvention, and many of the attendant advantages of the presentinvention, will become readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings in which likereference symbols indicate the same or similar components, wherein:

FIG. 1 illustrates an example three-dimensional (3-D) wafer-to-wafervertical stack forming a single chip;

FIG. 2 illustrates an example three-dimensional (3-D) wafer-to-wafervertical stack according to an embodiment of the present invention;

FIG. 3 illustrates an example silicon (Si) via etch in an examplethree-dimensional (3-D) wafer-to-wafer vertical stack according to anembodiment of the present invention; and

FIG. 4 illustrates an example process of forming one or more silicon(Si) vias in an example three-dimensional (3-D) wafer-to-wafer verticalstack according to an embodiment of the present invention.

DETAILED DESCRIPTION

The present invention is applicable for use with all types ofsemiconductor wafers and integrated circuit (IC) devices, including, forexample, MOS transistors, CMOS devices, MOSFETs, and new memory devicesand communication devices such as smart card, cellular phone, electronictags, gaming devices which may become available as semiconductortechnology develops in the future. However, for the sake of simplicity,discussions will concentrate mainly on exemplary use of a simplethree-dimensional (3-D) wafer-to-wafer vertical stack and an etch stoplayer for metallic wafer-to-wafer and die-to-die bonding in such athree-dimensional (3-D) wafer-to-wafer vertical stack, although thescope of the present invention is not limited thereto.

Attention now is directed to the drawings and particularly to FIG. 1, anexample three-dimensional (3-D) wafer-to: wafer vertical stack isillustrated. As shown in FIG. 1, the 3-D vertical stack (chip) 100 maycomprise any number of active device polysilicon (Si) wafers, such aswafer #1 110 including, for example, one or more microprocessors; wafer#2 120 including one or more memory devices; and wafer #3 130 includingone or more radio-frequency (RF) or optical communication devices. Thebottom wafer 110 is typically thick to support the stacking of the topwafers 120 and 130, while the top wafers 120 and 130 are thinned tominimize interconnection lengths between wafers 110, 120 and 130.

In a typical 3-D vertical stack 100 shown in FIG. 1, the active devicewafers 110, 120 and 130 are bonded using a dielectric layer 102, whileall active layers on wafers 110, 120 and 130 are electrically connectedusing interwafer vias 104. However, the dielectric layer 102 can becost-prohibitive for mass production. In addition, the interwafer vias104 between adjacent wafers 110, 120 and 130 can be lengthy which canlead to interconnect RC delays in active IC devices.

As a result, a metal to metal bond can be used to stack wafers 110, 120and 130 to form the vertical stack 100 shown in FIG. 1. This metal tometal bond method can serve not only as electrical connections to activeIC devices on the vertically stacked wafers 110, 120 and 130 on a 3-Dwafer-to-wafer vertical stack 100 but also bond adjacent wafers 110, 120and 130. Dummy metal bonding pads can also be made to increase thesurface area for wafer bonding and serve as auxiliary structures such asground planes or heat conduits for the active IC devices. In addition,improved etch stop layers for the Si via etch can be used in verticallystacked wafer processing (i.e., 3-D interconnect processing) whichprovide more efficient electrical conductivity between verticallystacked wafers 110, 120 and 130.

Turning now to FIG. 2, an example three-dimensional (3-D) wafer-to-wafervertical stack according to an embodiment of the present invention isillustrated. Specifically, FIG. 2 illustrates an example 2-wafervertical stack 200 according to an embodiment of the present invention,However, the number of wafers in a vertical stack is not limitedthereto. Through 3-D interconnect structure, wiring between verticallystacked wafers can be shortened resulting in a faster signal and minimalinterconnect RC delays. In addition, the vertical stack can effectivelyintegrate diverse process technologies on a single wafer process, suchas, for example, logic/memory stacking, processor stacking, opticalinterconnect, system-on-chip, and RF interconnect.

As shown in FIG. 2, the bottom silicon (Si) wafer 210 contains an activedevice layer 212 supporting one or more active IC devices (not shown).Likewise, the top Si wafer 220 also contains an active device layer 222supporting one or more active IC devices (not shown). The wafers 210 and220 may be aligned using a standard alignment tool and bonded, via ametal bonding layer 106 deposited on opposing surfaces of the bottomwafer 210 and the top wafer 220 at designated bonding areas to establishelectrical connections between active IC devices on vertically stackedwafers 210 and 220 and to bond adjacent wafers 210 and 220, whilemaintaining electrical isolation between bonding areas via an ILD layer108. The top wafer 220 can also be thinned by either a ChemicalMechanical Polish (CMP), grinding, or Silicon (Si) wet etch process soas to be much more pliable than those of standard thickness and to allowfor greater thickness variations across the vertically stacked wafers210 and 220 for the same applied bonding pressure.

In the example 2-wafer vertical stack 200 shown in FIG. 2, the metalbonding process between adjacent wafers 210 and 220 may be performed ina vacuum or an inert gas environment, and a dielectric recess can bemade surrounding the metal bonding areas to facilitate direct metalbonding between adjacent wafers 210 and 220 to ensure that the adjacentwafers 210 and 220 are bonded, while maintaining electrical isolationbetween the metal bonding areas. The metal bonding layer 106 may includea plurality of Copper (Cu) lines on opposing surfaces of the verticallystacked wafers 210 and 220 that can be used for metal diffusion bondingand serve as electrical contacts between active IC devices on thevertically stacked wafers 210 and 220. Copper (Cu) may be selectedbecause of its low electrical resistivity, high electro-migrationresistance and high diffusivity. However, other metallic materials canalso be used, including, for example, tin, indium, gold, nickel, silver,palladium, palladium-nickel alloy, titanium, or any combination thereof.

After the wafer bonding and silicon (Si) thinning processes arecompleted, one or more interwafer vias 224 may be etched, via the topwafer 220, to establish electrical connections of active IC devicesbetween vertically stacked wafers 210 and 220 and an externalinterconnect, via a C4 bump 226. Typically a tungsten “W” plug (metalcontact layer) is formed during a standard W contact process toestablish an interconnect with the copper (Cu) lines (the metal bondinglayer 106) of the vertically stacked wafers 210 and 220. The “W” plugmay then serve as an etch stop to stop the silicon (Si) via etch. Inaddition, existing barrier layers such as TaN/Ta (TantalumNitride/Tantalum), which form a barrier layer for the copper (Cu)interconnect, or TiN/Ti (Titanium Nitride/Titanium), which form anadhesion layer for the W plug, can be used to stop the silicon (Si) viaetch during the Si via etching process. However, both the TaN/Ta and theTiN/Ti layers do not constitute a good etch stop layer because Si etchchemistry can easily etch them. As a result, W plug can be recessed andCu lines can be oxidized, which may negatively impact the power deliverysystem. Accordingly, NiSi (Nickel Silicide) has been advantageouslyutilized to serve as an etch stop layer for Si via etch in thevertically stacked wafers 210 and 220 to provide electrical conductivitybetween vertically stacked wafers 210 and 220. NiSi (Nickel Silicide)may be selected because of its high selectivity with the Si via etch andits low resistance to prevent any negative impact on the power deliverysystem. A several micron deep Si via etch can stop at a thin NiSi etchstop layer very easily. Another advantage of using NiSi as an etch stoplayer is that, if NiSi is used as a Silicide material in transistors ofIC active devices, then no additional process steps are required to forma NiSi layer.

Turning now to FIG. 3, an example silicon (Si) via etch in an examplethree-dimensional (3-D) wafer-to-wafer vertical stack 200 according toan embodiment of the present invention is illustrated. As shown in FIG.3, a tungsten “W” contact plug 310 is formed during a standard W contactprocess. A nickel silicide (NiSi) etch stop layer 320 is formed beforethe W contact plug 310 is established and before the wafers 210 and 220are bonded. The Si via etch can be stopped at the NiSi etch stop layer320 to prevent etching through the W contact plug 310. Each of theinterwafer vias 224 may contain an oxide layer 332 deposited on asidewall, a copper (Cu) barrier/seed 334 deposited on the oxide layer332 and the NiSi etch stop layer 320, and copper (Cu) 336 electroplatedon the Si via 224 to serve as an electrical connection between active ICdevices located on the vertically stacked wafers 210 and 220 and anexternal interconnect.

FIG. 4 illustrates an example process of forming one or more silicon(Si) vias in an example three-dimensional (3-D) wafer-to-wafer verticalstack according to an embodiment of the present invention. After thetungsten “W” contact plug 310 and NiSi etch stop layer 320 are formed atthe top Si wafer 220, and the wafer-to-wafer bonding and silicon (Si)thinning processes are completed, interwafer vias 224 (see FIGS. 2-3)are patterned by conventional lithography and the silicon (Si) is etchedusing an etch mask until stopped by the NiSi etch stop layer 320 atblock 410. After the interwafer vias 224 are etched, an oxide layer 332as shown in FIG. 3 can be deposited in the Si vias 224 at block 420.Then anisotropic oxide etch is done to remove the oxide at the bottom ofthe Si vias 224 at block 430. After the anisotropic oxide etch, abarrier/seed layer 334 is then deposited on the oxide layer 332 and thebottom of the Si vias 224 at block 440. After the barrier/seed layer334, copper (Cu) can then be deposited in the Si vias 224, viaelectroplating and Chemical Mechanical Polish (CMP), to establishelectrical connections between active IC devices located on thevertically stacked wafers 210 and 220 and an external interconnect, viathe C4 bump 226 shown in FIG. 2.

As described in this invention, the NiSi etch stop layer according to anembodiment of the present invention can effectively enable the silicon(Si) via etch to provide electrical conductivity between verticallystacked wafers without undesirable copper (Cu) oxidation or tungstenrecess. If NiSi is already used as a silicide material in transistors ofIC active devices, then no additional process steps may be required toform NiSi. The existing silicide material can be effectively used as anetch stop during the formation of one or more interwafer vias.

While there have been illustrated and described what are considered tobe exemplary embodiments of the present invention, it will be understoodby those skilled in the art and as it technology develops that variouschanges and modifications may be made, and equivalents may besubstituted for elements thereof without departing from the true scopeof the present invention. For example, other types of metallic materialsthat exhibit a high selectivity with the silicon (Si) via etch may alsobe used in lieu of NiSi (Nickel Silicide) to serve as an etch stoplayer. Many modifications may be made to adapt the teachings of thepresent invention to a particular situation without departing from thescope thereof. Therefore, it is intended that the present invention notbe limited to the various exemplary embodiments disclosed, but that thepresent invention includes all embodiments falling within the scope ofthe appended claims.

What is claimed is:
 1. A method of forming a silicon (Si) via invertically stacked wafers having a contact plug extending from selectedmetallic lines of a top wafer and an etch stop layer processed beforethe contact plug, comprising: selectively etching through a silicon (Si)of the top wafer until stopped by the etch stop layer comprised ofnickel silicide (NiSi) to form a Si via; depositing an oxide layer toinsulate a sidewall of the Si via; forming a barrier layer on the Sivia; and filling the Si via with a conduction metal to provideelectrical connection between active IC devices located on verticallystacked wafers and an external interconnect.
 2. The method as claimed inclaim 1, wherein the selected metallic lines are copper (Cu) linesdeposited to serve as electrical contacts between active IC devices onthe vertically stacked wafers.
 3. The method as claimed in claim 1,wherein the contact plug is comprised of tungsten (W).
 4. The method asclaimed in claim 1, wherein the conduction metal deposited in the Si viais copper (Cu).
 5. The method as claimed in claim 1, wherein thevertically stacked wafers are bonded via respective metallic lines whichserve as electrical contacts between active IC devices on the verticallystacked wafers.
 6. The method as claimed in claim 1, wherein each of thevertically stacked wafers contains an active layer supporting one ormore active IC devices.
 7. The method as claimed in claim 1, wherein theetch stop layer is formed or deposited in designated areas along withthe contact plug before the metallic lines are selectively deposited onthe top wafer.
 8. A method of forming vertically stacked wafers,comprising: forming an etch stop layer comprised of nickel silicide(NiSi) on designated areas on a surface of a top wafer; forming acontact plug layer on the etch stop layer on the designated areas on thesurface of the top wafer; forming vertically stacked wafers by bondingthe top wafer with a bottom wafer, via respective metallic lines; andforming an interwafer via on the top wafer by selectively etchingthrough the silicon (Si) of the top wafer until stopped by the etch stoplayer comprised of nickel silicide (NiSi), depositing an oxide layer toinsulate a sidewall of the interwafer via, depositing a barrier layer onthe oxide layer and on the bottom of the interwafer via, and filling theinterwafer via with a conduction metal to provide electrical connectionbetween active IC devices located on vertically stacked wafers and anexternal.
 9. The method as claimed in claim 8, wherein the respectivemetallic lines are copper (Cu) lines deposited on opposing surfaces ofthe vertically stacked wafers to serve as electrical contacts betweenactive IC devices on the vertically stacked wafers.
 10. The method asclaimed in claim 8, wherein the contact plug layer is comprised oftungsten (W).
 11. The method as claimed in claim 8, wherein theconduction metal deposited in the Si via is copper (Cu).
 12. The methodas claimed in claim 8, wherein each of the vertically stacked waferscontains an active layer supporting one or more active IC devices.
 13. Amethod for forming a three-dimensional (3-D) vertically stacked waferstructure, comprising: forming a first wafer including an active layerto support one or more integrated circuit (IC) devices; forming a secondwafer including an active layer to support one or more integratedcircuit (IC) devices; depositing metallic lines on opposing surfaces ofthe first and second wafers at designated areas to establish metalbonding between the first and second wafers in a stack and electricalconnections between active IC devices on the first and second wafers inthe stack; and forming one or more interwafer vias within the firstwafer, to provide electrical connections between active IC devices onthe first and second wafers in the stack and an external interconnect,wherein each of the interwafer vias is formed by: selectively etchingthrough the top wafer until stopped by an etch stop layer comprised ofnickel silicide (NiSi); depositing the oxide layer to insulate asidewall of the interwafer via; depositing a barrier layer on the oxidelayer and on the bottom of the interwafer via; and filling theinterwafer via with the conduction metal to provide electricalconnections between active IC devices on the first and second wafers inthe stack and the external interconnect.
 14. The method as claimed inclaim 13, wherein the metallic lines include copper (Cu) lines depositedon opposing surfaces of the first and second wafers to serve aselectrical contacts between active IC devices on the first and secondwafers.
 15. The method as claimed in claim 13, wherein the contact plugis comprised of tungsten (W).
 16. The method as claimed in claim 13,wherein the conduction metal deposited in the interwafer vias is copper(Cu).
 17. The method as claimed in claim 13, wherein the etch stop layercomprised of nickel silicide (NiSi) is formed or deposited in designatedareas along with the contact plug before the metallic lines areselectively deposited on the top wafer.
 18. A method of forming aninterwafer via in a three-dimensional (3-D) vertically stacked waferstructure having vertically stacked wafers, comprising: selectivelyetching through a top wafer until stopped by an etch stop layercomprised of nickel silicide (NiSi) to form an interwafer via;depositing an oxide layer to insulate a sidewall of the interwafer via;depositing a barrier layer on the oxide layer and on the bottom of theinterwafer via; and filling the interwafer via with the conduction metalto provide electrical connections between active IC devices on thevertically stacked wafers and an external interconnect.
 19. The methodas claimed in claim 18, wherein the vertically stacked wafers are bondedvia respective metallic lines which serve as electrical contacts betweenactive IC devices on the vertically stacked wafers.
 20. The method asclaimed in claim 19, wherein the etch stop layer is formed or depositedon designated areas along with a contact plug before the metallic linesare selectively deposited on the top wafer.